I'm the front office manager at a furniture store. I watch salespeople skitter around all day trying to figure out financing and get the best deals and make furniture fit. Their most-used accessory is their tape measures. One of my tiny joys is watching them try to measure something from an awkward angle and seeing the tape measure snap back and smack them in the hand/face.

So the basic idea is a tape measure with no upper limit for stretching out and that no one can get a grip on, so it's constantly smacking people in the face. Originally people think it's just an infinite tape measure so they can loan it out for home projects, people play balancing contests with it, etc. Slowly all these things get suspended because people keep hitting themselves.

I know, it's pretty silly, but I also think it's kind of funny. Does anyone else?

Ah, bonjour, Ahbonjour!
I also think it's funny. I do believe you should go crazy with this idea, perhaps… Take it slow at the start and then pretend to make it Keter or something. Could be a good SCP-J. Is this what you had in mind?

To both of you: there are so many possibilities! Yes, the tape anomaly could be discovered directly by the Foundation building personnel; or it could create a huge bit of havoc in a store like the one you work in IRL, Ahbonjour, and then…
It could develop an angry mind of its own from all the swearing of the personnel who got hurt…
Someone a bit silly could try to measure the circumference of the Earth with it… Can you imagine the force of the snapback if the guy gets from Quito, Ecuador to Singapore before letting go?
(and also imagine the logistics of such an undertaking… Follow the equator through the Andes and the Amazon area from Quito to Brazil, book a trip by ship across the Atlantic, then land again, then….!)
I'm already chuckling at the thought…!